Alkali metal leak detector

ABSTRACT

A layer of refractory fiber insulation such as a felt sheet consisting essentially of aluminum silicate fibers is placed between two layers of conducting material, such as steel screens which are connected in an electrical circuit to a detecting device, such as an electric light. This device is a leak detector in which a drop of liquid sodium, potassium or other alkali metal will short out the screens and give a signal on the detecting device.

United States Patent 1191 Niemoth A l 1March 20, 1973 ALKALI METAL LEAKDETECTOR [75] Inventor: Hubert R. Niemoth, Elinhurs tT [73] Assignee:United States of America as represented by theUnited States AtomicEnergy Commission 22 Filed: Oct.6, 1971 [21] Appl. No.: 187,043

[52] U.S. Cl. ..340/242, 731/491, 200/6105 [51] Int. Cl. ..G08b 21/00[58] Field of Search ..340/242; 73/40, 40.5, 46, 49.1,

73/49.2, 49.3; ZOO/61.04, 6105; 137/312;

[56] I References Cited UNITED STATES PATENTS FOREIGN PATENTS ORAPPLICATIONS 1,016,928 1/1,966 Great Britain .l73/40.5 R 1,212,3273/1960 France ..73/40 1,377,519 9/1964 France ..73/40.5 R

Primary Examiner-John W. Caldwell Assistant Examiner-Daniel MyerAttorney-Roland A. Anderson 57 ABSTRACT A layer of refractory fiberinsulation such as a felt sheet consisting essentially of aluminumsilicate fibers is placed between two layers of conducting material,such as steel screens which are connected in an electrical circuit to adetecting device, such as an electric light. This device is a leakdetector in which a drop of liquid sodium, potassium orother alkalimetal will short out the screens and give a signal on the detectingdevice.

7 Claims, 2 Drawing Figures 7 sop/0M WETTABLE ALKALI METAL LEAK DETECTORCONTRACTUAL ORIGIN OF THE INVENTION The invention made herein was madein the course of, or under, a contract with the United States AtomicEnergy Commission.-

BACKGROUND OF THE INVENTION metal which will bring about corrosion. Suchcorrosion in time causes a catastrophic failure of either or both thealkali metal filled pipes or the steam pipes of the reactor. The earlydetection of an alkali metal leak is, therefore, very important. 7

Various devices and methods of detecting sodium leaks have been used.These include sodium collector- I type leak systems, systems of sodiumleakage into a liquid, and sodium ignition systems in which the fumes,

- smoke or flame from the leaked sodium is detected.

Another method involves the use of a grid of anodized aluminum wireplaced below the heat exchangers' The resistance between theintersecting wires is measured, a significant decrease being indicativeof failure of the insulating film at the intersections. Thesepriordevices and methods are expensive and not entirely reliable.-

An object of this invention is to provide a simple, inexpensive andreliable device and method for detecting liquid alkali metal leaks.

Other objects and advantages will become apparent as the followingdescription progresses, reference being had to the'accompanying drawingwherein FIG. 1 is a cross section of a pipe incorporating the device ofthis invention and FIG. 2 shows an alternative embodiment.

SUMMARY OF THE INVENTION In accordance with this invention an insulatinglayer of a composition comprising sodium wettable refractory'fibers,such as a layer of a composition consisting essentially of aluminumsilicate fibers, is. placed between two layers of conducting materialwhich are connected in an'electrical circuit to a device for detectingelectric current. This device is a leak detector in which a drop ofliquid sodium, potassium or other alkali metal will short out the twoconducting layers and give a signal on the detecting device.

The insulating layer is preferably in the form of a felt sheet ofaluminum silicate fibers. The sheet may contain other material, such asminor amounts of boron oxide, aluminum oxide and sodium silicate binder.Aparticularly satisfactory insulating felt is that sold by RefractoryProducts Co. underthe name of WRPX AQ Felt. This material consistsessentially of short aluminum silicate fibers felted into a sheet. Itcontains a small amount of water which is baked out before use.

A layer of aluminum silicate fiber batting or a woven fiber sheet mayalso be used as the insulating layer.

The conducting layers may be in the form of a metal screen, such as astainless steel screen wire. One of the conducting layers may be themetal pipes containing the sodium or other alkali metal.

In making the detector device, a layer of the wet insulating sheet issandwiched between two sheets of the conducting material, which are usedas the two contacts to an electrical indicating circuit.

The assembly is baked out to remove the moisture that is generallypresent in the insulation sheet.

With aluminum silicate fiber sheets, an insulating sheet having adielectric constant at C of 4.81 at 100 cycles is readily attained.

The aluminum silicate fiber insulation is not softened by water and itsacid and chemical resistance is good. It is attacked by alkali.

While I do not wish to be restricted to any theory as to the mechanismof the action of my device or reason for its fast and reliable action,it is believed that the aluminum silicate fibers, particularly in feltform, are readily wettable by sodium which, on leakage from thesurrounding pipe or container, is drawn by capillary action through theinsulating layer sothat both conductto the leak area will exist whichproduces fast response.

With wire screens, the close weave spacing between ing layers arequickly contacted by the sodium thus closing the electrical circuit. andgiving a signal for the leak. Also, it is believed that the action isaccelerated by the fact that the sodium or other alkali metal attacksthe insulation and eventually destroys it.

The whole indicator assembly can be molded'around pipes or valves withease, so that very close proximity wires makes for more'completecoverage of the entire area. v I

The detectors may also be tailored to large or small areas and remotelymonitored. For example, there may be a plurality of patches of steelscreen, each in contact with selected portions of the exposed surface ofthe refractory felt insulation with means for electrically connectingeach of said patches to a remote detector.

Any type of sodium or potassium wettable refractory fiber may be usedincluding quartz fibers and zirconium silicate fibers, but these, whileoperable, are not preferred and do not have all the advantages of thealu minum silicate fibers'for obtaining quick and reliable leakdetection for hot liquid alkali metal leakage.

DETAILED DESCRIPTION OF THE INVENTION An illustrative embodiment of theinvention is shown in the accompanying drawing in FIG. 1 of which a thinaluminum silicate felt sheet of inch thickness 1 is wrapped around ametal pipe 2 of the heat exchange loop of a liquid metal fast breedernuclear reactor (LMFBR). A stainless steel close woven metal screen 3was then wrapped around and in close contact with the insulator feltsheet. The assembly was baked to remove moisture from the insulatingsheet and? gave a dielectric constant at 25C of 4.81 at 100 cycles.

In the device shown in the drawing the electrical circuit 4 providedwith power supply 6 causes the light bulb S to light when liquid alkalimetal leaks through the pipe walls 2 and through the felt I to contactthe metal screen 3.

Various types of signaling devices other than light The embodiments ofthe invention in which an exclusive property or privilege is claimed aredefined as follows:

1. An alkali metal leak detector comprising an insulating sheetconsisting essentially of sodium wettable refractory fibers in contacton each side of the sheet with electricity conducting layers, and meansfor detecting the completion of an electrical circuit betweensaidconducting layers when shortedby liquid alkali metal.

2. A leak detector in accordance withclaim l in which the sodiumwettable refractory fibers are aluminum silicate fibers.

3. A leak detector in accordance with claim 2 in which the sheet is afelt sheet.

4. A leak detector in accordance with claim 3 in which at least one ofthe conducting layers is a steel screen.

5. A leak detector in accordance with claim 1 in which at least one ofthe conducting layers is a metal screen.

6. An alkali metal leak detector comprising a thin layer sodium wettablerefractory felt insulation molded against the surface of a container tobe monitored, said container being an electrical conductor, a pluralityof patches of a conducting screen each positioned against selectedportions of the exposed surface of said insulation, and means forelectrically connecting each of said patches to a remote detector, saiddetector being responsive to a circuit completion caused by contact ofliquid alkali metal with any of said patches.

7. A leak detector in accordance with claim 6 in which the refractoryfelt comprises fibers of aluminum silicate. i

2. A leak detector in accordance with claim 1 in which the sodiumwettable refractory fibers are aluminum silicate fibers.
 3. A leakdetector in accordance with claim 2 in which the sheet is a felt sheet.4. A leak detector in accordance with claim 3 in which at least one ofthe conducting layers is a steel screen.
 5. A leak detector inaccordance with claim 1 in which at least one of the conducting layersis a metal screen.
 6. An alkali metal leak detector comprising a thinlayer sodium wettable refractory felt insulation molded against thesurface of a container to be monitored, said container being anelectrical conductor, a plurality of patches of a conducting screen eachpositioned against selected portions of the exposed surface of saidinsulation, and means for electrically connecting each of said patchesto a remote detector, said detector being responsive to a circuitcompletion caused by contact of liquid alkali metal with any of saidpatches.
 7. A leak detector in accordance with claim 6 in which therefractory felt comprises fibers of aluminum silicate.